This invention relates to aluminium reduction cells, and particularly to the problem of cathode current collection therein. These cells are of the kind in which the electrolyte comprises molten cryolite Na.sub.3 AlF.sub.6 containing dissolved alumina Al.sub.2 O.sub.3, and electrolysis is performed between an anode suspended in the electrolyte and a cathode at the floor of the cell. In conventional cells, the floor is of carbon in which are embedded steel members connected to the external electricity supply. The carbon potlining transmits the electric current to the steel connecting members; but carbon is a rather poor electrical conductor, with the result that the cell voltage is higher than would be the case if a better cathode current collector were used.
U.S. Pat. No. 3,093,570 (Dewey) and British Patent Application No. 2065174 (Odek) both show cathodes of titanium diboride TiB.sub.2 mounted in aluminium slabs for connection to the external electricity supply. TiB.sub.2 is a better electrical conductor than carbon; but it is expensive and difficult to form, and has low mechanical strength and a coefficient of thermal expansion very much greater than that of carbon or alumina or other potlining material.
Due partly to the penetration by electrolyte, which inevitably occurs, the lining of a cell is generally not dimensionally stable. During the course of a cell campaign, the lining usually expands, but the expansion is not uniform. As a result, cathode current collectors embedded in the cell floor and walls are subjected to compressive forces and to shear forces in an unpredictable manner. Monolithic TiB.sub.2 current collectors react to such forces by breaking, which may severely affect their current-carrying ability. For all these reasons, solid TiB.sub.2 cathodes have not achieved any significant commercial success.
It would be convenient and cheap to use cathode current collectors of aluminium metal. The fact that aluminium melts (660.degree. C.) far below the normal cell operating temperatures (950.degree.-980.degree. C.) means that the high-temperature end of such collectors would be fluid, but that does not in principle make them unsuitable. In practice however, it is found that thermal convection and magnetic effects cause efficient stirring of the molten metal. This results in downward movement of the solid-liquid boundary and an unwanted increase in thermal conduction. Also, crystallisation of alumina, cryolite and other phases can occur in the molten metal. For these reasons such collectors cannot be used unless special precautions are taken.
In U.S. Pat. No. 3,607,685 (Johnson) there are described various designs of cathode current collector which are intended to overcome these difficulties. One design comprises an outer refractory tube containing a number of parallel spaced refractory rods or fibres surrounded by molten aluminium; the rods or fibres, which are intended to restrain molten metal circulation, may be made of or coated with a material which is wet by aluminium metal. Another design uses aluminium alloys that have higher melting points and higher viscosities than commercial primary aluminium. Yet another design uses conductor assemblies each comprising a refractory tube and an aluminium core conductor, the high-temperature end of each being positioned at the bottom of a bowl-shaped depression in the cell potlining.
All the designs in the Johnson patent are characterized by the fact that the section of the cathode current collector adjoining the molten aluminium cathode (the pad) is molten and mixes with the pad. We have found that this use of molten metal as the conductor of electricity in the upper section of the collector places severe restrictions on the design and positioning of the collector. Collectors of this kind only work well if the associated magnetic fields are kept to a minimum. Unless this is done, and unless the cross-section of the collector is kept at a low level, magnetic stirring generates high molten metal velocity resulting in the problems noted above.
Johnson suggests the use of refractory rods or fibres in the molten metal to counteract magnetic stirring. He also notes that the use of granular refractory material is not effective for the purpose. But we have found that it is in practice difficult to design cathode current collectors which contain enough (non-conducting) refractory material to counteract magnetic stirring, while nevertheless containing enough molten metal to provide sufficient electrical conductivity.